Typically, speech coding makes use of vocal tract modeling to reconstruct or synthesize the speech signal so that it resembles as close to the original as possible. Such speech coding includes adaptive multi rate wideband (AMR-WB) speech coding which is used in the 3GPP system (see Non-Patent Document 1). This AMR-WB speech coding was also selected and approved by the ITU-T as ITU-T recommendation G.722.2 (Non-Patent Document 2). Hereinafter, a case will be described as an example where AMR-WB speech coding at a bit rate of 23.85 kbit/s is used.
One of the important blocks of AMR-WB speech coding is a fixed codebook search (FIG. 1). In AMR-WB speech coding, each frame of two hundred and fifty six downsampled speech samples is divided into four subframes of sixty four samples each. During the fixed codebook search, the subframe is divided into four tracks. For mode 8 of AMR-WB speech coding, for each track, six pulse positions are selected from among the sixteen possible pulse positions in each track. That is, the number of pulses for each subframe is set to twenty four from p0 to p23. These twenty four pulse positions from p0 to p23 are encoded to form a codebook index which is used for synthesizing the speech for each subframe (see Non-Patent Document 1).
Presently, ITU-T recommendation G.722.2 supports AMR-WB speech coding for monaural signals, but does not support AMR-WB speech coding for stereo speech signals.
With development of a wide transmission band in mobile communication and IP communication and diversification of services in such communications, high speech quality and high-fidelity speech communication are demanded. For example, from now on, it is expected to increase demand of communication in a hands free video telephone service, speech communication in video conference, multi-point speech communication where a plurality of callers hold a conversation simultaneously at multiple locations and speech communication capable of transmitting the sound environment of the surroundings with high fidelity. In this case, it is desired to implement speech communication using stereo speech that has high fidelity compared to monaural signals and that makes it possible to identify the locations of a plurality of callers. To implement speech communication using stereo speech, coding of stereo speech signals is essential. Methods of coding stereo speech signals include independently coding a speech signal of each channel (dual-monaural coding).    Non-Patent Document 1: “AMR Wideband Speech Codec; General Description”, 3GPP TS 26.171, V5.0.0 (2001-03)    Non-Patent Document 2: “Wideband Coding of Speech at Around 16 kbit/s Using Adaptive Multi-Rate Wideband (AMR-WB)”, Geneva, ITU-T Recommendation G.722.2 (2003-07)